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c#castingpolymorphism
RayOldProf
RayOldProf

Reputation: 1070

Polymorphism and explicit Casting

I have the following architecture(the analogy sucks but W/E).

enter image description here

In program and other logic classes I have lots of methods that uses the specific type of the finger (MonkeyFinger). This mean that i have to explicitly cast in all those testMethods.

Is there any design pattern/solution to avoid explicit casts?

EDIT Code: 

Monkey govi = new Monkey(...)
Program test = new Program()
test.testFinger1((MonkeyFinger) govi.GetHand.getFinger)

...

Upvotes: 0

Views: 281

Answers (3)

Rob
Rob

Reputation: 27357

You can try something like this:

public class Animal<TFingerType> where TFingerType : IFinger
{
    Hand<TFingerType> GetHand() 
    {
        //... Do something
    }
}

public class Monkey : Animal<MonkeyFinger> { }
public class Hand<TFingerType> where TFingerType : IFinger
{

}
public interface IFinger
{

}
public class MonkeyFinger : IFinger {
}

At least in your given example, it doesn't make sense for a Monkey to return a hand which contains HumanFingers. The hand itself is really defined by what type of fingers it has.

Then your code becomes:

Monkey govi = new Monkey(...)
Program test = new Program()
test.testFinger1(govi.GetHand.getFinger() /* getFinger here returns a MonkeyFinger */)

Note that the fingers are still IFingers, and can be used in that context, but this approach also provides concretely typed fingers.

Upvotes: 5

Vadim Martynov
Vadim Martynov

Reputation: 8892

Is there a real need to pass concrete finger to test method? When you are using interface you are define a contract which each implementation should follow. You can extend parent behavior in child class (or replace it but it's not correspondent with Liskov substitution principle) with method overriding but you test only contract then why do you need to pass MonkeyFinger insted of IFinger in test method?

Upvotes: 0

Patrick Hofman
Patrick Hofman

Reputation: 156938

I think it is best to create a method to do the checking for you. And yes, the casting is necessary if you want to test on some assumptions (like a monkey having only monkey fingers).

Something like:

public static T TestAndConvert<T>(object o)
{
    Assert.IsInstanceOfType(o, typeof(T));

    return (T)o;
}

Here you first check if the type is correct and then you return a typed instance. This way you are sure the type is correct and you have proper testing.

Use it in your test calls:

testFinger1(TestAndConvert<MonkeyFinger>(finger));

(From your diagram I am not sure if you use an automated test framework, like Unit Tests in Visual Studio, I recommend to do so)

Upvotes: 0

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